Ch.1-5: Nature of Science, Microscope, Cellular organization, Enzyme, Cell Membrane

Question 1

1. Describe and explain the importance of fluid mosaic model (3 marks)

Answer 1

• Fluidity: the fluid nature of phospholipid bilayer, allows the membrane to break and fuse readily, which allows phagocytosis and exocytosis to occur (1)
• Mosaic: protein molecules are interspersed among and embedded on, through and between the phospholipid bilayer, which serves as channel proteins for the transportation for hydrophilic substances across the membrane (1)
• Some proteins serve as antigens for cell recognition, or as receptor site for receiving chemical messenger (1)

Question 2

2. Describe how control set-up are set up, and explain why it is necessary to have such a set-up (2 marks)

Answer 2

• The control set-up should be exactly the same with the experimental set-up, except removing the independent variable (1)
• It is necessary because it can eliminate the environmental factors that may affect the experimental result, by comparison of the two set ups, and so the results of the control can be taken as a standard(1)

Question 3

3. Explain the effects of submerging a plant cell into distilled water (4 marks)

Answer 3

• The water potential of the cytoplasm is lower than that of the distilled water (1)
• Hence there will be a net movement of water moving out of the cell by osmosis (1)
• As water is lost in the cell, the volume of the cytoplasm shrinks (1)
• And finally this causes the cell membrane to be pulled away from the cell wall, and the cell become plasmolysed (1)
(*Vacuole is transparent and invisible!!)

Question 4

1. A is chopped into pieces, while B is not. Explain the difference in enzyme activity between the reactions of A and B (2 marks)

Answer 4

• A has a higher surface area in contact with the enzyme than B, as it is chopped into smaller pieces/ cubes (1)
• This increases the chance of successful and effective collision between the enzymes and A to form enzyme-substrate complex, which is higher than that of B. (1)

Question 5

2. Given that Beetroot contains a red pigment within the vacuole. It is chopped into smaller pieces and also is put into a water bath with a higher temperature. Explain why the solution around it became red in colour (3 marks)

Answer 5

• The vacuole membrane is damaged, causing the leakage of red pigment into the solution by: (1)
• First, the physical damage from chopping the beetroot into smaller cubes (1)
• Second, the increase in temperature causing the denature of membrane proteins and increase in fluidity of phospholipids, leading to the vacuole membrane to be fully permeable (1)

Question 6

3. It is known that apple will turn brown by the action of a specific enzyme, but by sprinkling lemon juice on the cut surface, the rate is slower. Explain the biological principle. (3 marks

Answer 6

• When lemon juice is added, the pH at the apple surface is lowered (1)
• Hence, this causes the conformation of the active site of the enzyme to be altered, which the specific substrates can no longer fit into the active sites (1)
• No enzyme-substrate complex can be formed, and hence there will be no enzymatic reactions occurring (1)

Question 7

4. State and explain the effect of temperature on the activity of enzyme P when the temperature increases from 20-80 degrees (3 marks)

Answer 7

• The enzymatic activity increases as temperature increases between 20-80C (1)
• It is because the kinetic energy possessed by the substrate and enzyme molecules increases (1)
• Which increases the chance of effective collision between the enzyme and substrate to form enzyme-substrate complexes (1)

Question 8

1. State two functions of carbohydrates (2 marks)

Answer 8

• First, carbohydrates are the main source of energy, as glucose is a raw material for carrying out respiration, which releases energy in the for of ATP (1)
• When carbohydrates are in excess, they can be converted to glycogen or lipids and are stored in the body as adipose tissue or subcutaneous fat as energy reserves (1)

Question 9

2. State the functions of vitamins and dietary fibres (2 marks)

Answer 9

• Vitamins regulate various metabolic activities (1)

* While dietary fibres add bulk to food to stimulate peristalsis (and also hold water in faeces and keep it soft (1)

Question 10

3. Why is photosynthesis so important for a plant? (3 marks)

Answer 10

• Photosynthesis produces glucose as the main product (1)
• Glucose is a raw material for the release of energy by aerobic respiration for the growth and development of the plant (1)
• While glucose can also be synthesized to form cellulose, which is essential for the formation of cell wall (1)

Question 11

1. Some people does not have lactase in their alimentary canal. Explain how consuming lactose will lead to diarrhea (4 marks)

Answer 11

• Lactose cannot be further broken down into glucose due to the lack of lactase, and hence lactose is remained in the small intestines, and cannot be absorbed via intestinal cells due to its large molecular size (1)
• The presence of lactose lowers the water potential of chyle (1)
• Water is drawn into chyle by osmosis
• The large volume of fluid in chyle stimulates peristalsis (1), leading to diarrhea

Question 12

2. Patient A has less villi, compared to a healthy individual. Explain how this reduces the digestive ability of the patient (3 marks)

Answer 12

• Carbohydrase and proteases are present on the cell membranes of the epithelial cells of intestinal wall (1)
• With a reduced number of villi, this reduces the surface area for these enzymes to embed onto the epithelial cells (1)
• which leads to a lower efficiency and poor digestion of carbohydrates and protein (1)

Question 13

3. Describe the role of pancreas for the digestion of lipids (3 mark)

Answer 13

• Firstly, pancreas secretes pancreatic juice, which contains pancreatic lipase and sodium hydrogencarbonate (1)
• Pancreatic lipase catalyses the breakdown of lipid into fatty acids and glycerol (1)
• The sodium hydrogencarbonate provides an alkaline medium that is close to the optimum pH of pancreatic enzymes, to increase the rate of digestion (1)

Question 14

4. Describe the role of liver for the digestion of lipid (3 marks)

Answer 14

• The liver produces bile, which contains bile salt (1)
• Bile salt emulsify lipid and breaks lipid into smaller oil droplets, so to increase the surface area to volume ratio for lipase to act on (1)
• And ultimately, increase the rate of enzymatic activity (1)

Question 15

5. Patient A results in part of the stomach being removed. Explain how this may lead to poor digestion of food (4 marks)

Answer 15

• Firstly, as the volume of stomach is reduced, there will be less churning to break down food into smaller pieces (1)
• Hence, the surface area of food in contact with eh digestive enzymes are reduced, which hence lowers the enzymatic activity and leads to poor digestion of food (1)
• Secondly, less protease is present for breaking down proteins into peptides (1)
• More proteins remain undigested in the stomach (1)

Question 16

6. State and describe the function of gall bladder in digestion (2 marks)

Answer 16

• Gall bladder acts as a temporary storage of bile (1)

* Which regulates the release of bile into the duodenum (1)

Question 17

7. Explain three structural adaptations of how the small intestine is adapted to the absorption of digested food substances (6 marks)

Answer 17

• First, there are a large number of villi, and each villus is a finger-like projection (1)
• This increases the surface area for a higher rate of absorption of digested food substances (1)
• The wall of villus is very thin (1)
• which shortens the distance for transport of digested food substances by diffusion, increasing the absorption rate (1)
• There is a dense network of blood capillaries in villi (1)
• Which transports the digested substances in the blood away quickly, maintaining a steep concentration gradient of digested substances between the blood and the chyle by diffusion (1)